1. Field of the Invention
The invention relates to molded composite panels formed from a layer of mineral fibers, a surfacing layer of synthetic foam, and, optionally, a surfacing film to provide a desired appearance of, for example, a woven fabric.
2. Discussion of the Background
Composite panels which can be used particularly as interior headlinings for motor vehicles are generally made of a layer of bonded mineral fibers and a surfacing layer, for example of synthetic foam. The surfacing layer serves as a layer to be glued to a woven fabric covering layer. A sheet of non-woven material may likewise be glued onto the layer of mineral fibers, on the surface opposite the finishing layer and not visible after assembly. This sheet protects the mineral fibers during the various operations needed for positioning the finished product.
Such panels are preferably made by a so-called single-pass process described in Inventors' European Patent Applications EP Nos. 0 112 210 and EPO 124 387 (=U.S. Pat. No. 4,609,519). In this process a mat of mineral fibers, particularly non-bonded mineral fibers, is impregnated with a hot polymerizing resin, preferably by spraying, after which it is cut into portions of desired lengths. An intermediate layer of, for example, synthetic foam and a covering appearance-imparting layer is then stacked on each portion, with interposed films of suitable adhesives. The assembly constitutes an original product which is hot pressed in order to polymerize the resin which bonds the mineral fibers and which also activates the films of adhesive substances.
Generally, the press consists of a mold and a counter-mold. The shapes of this mold and counter-mold are such that the composite panel has thinner portions, particularly around the edge and possibly on any areas which subsequently have to be recessed. This is envisaged, for instance, for the accommodation of a cailing lamp.
Since the basic product is of a constant thickness, these thinner portions have greater density which improves the rigidity of the composite panel, facilitates assembly operations, and in particular facilitates any drilling needed for assembly. Furthermore, thinned edges make it possible for the composite panel to be fitted without a frame, leaving the edge visible.
This type of installation is more particularly desirable if the vehicle has flush fitting windows, which are simply glued into place and which are therefore not surrounded by frames which mask the front edge of the headlining. Likewise, the recessed zones have to have very clean edges to allow edge-to-edge fixing without any framing.
One limitation of the single-pass process described above is that such clean and thinned edges can only be obtained by using intermediate layers of small thickness. If the molding is carried out at a high temperature so that the mold and counter-mold are at temperatures exceeding 180.degree. C., for example, then the intermediate layer of synthetic foam is completely deteriorated by the heat and then only serves as an underlayer to assist gluing of the covering fabric. This gluing underlayer is quite advantageous because it imparts to the product an excellent surface condition but it can be readily appreciated that it is pointless to have thick underlayers which would in any case be reduced to the thickness of a film after the surfacing.
On the other hand, if the molding operation is carried out at a relatively low temperature, particularly with a temperature of, for instance, around 150.degree. C, especially for the counter-mold which is in contact with the surfacing sheet, then the films of adhesive will be activated but this will not cause the covering fabric to shine nor will there be any thermal degradation of the synthetic foam. But in this case, the intermediate layer of foam is only provisionally compressed and, once the molding operation ceases, it resumes its initial thickness.
The composite panel therefore comprises portions which are easily made thinner by reason of the final compression of the layer of mineral fibers but from an aesthetic point of view, the inefficiency of these thinned portions is obvious for composite panels of which the nominal thickness is, for instance, 13 mm, distributed between 5 mm of mineral fibers and 8 mm of synthetic foam, while over the thinned portions this thickness is still 10 mm, distributed between 2 mm of mineral fibers and 8 mm of synthetic foam.
Such thicknesses of synthetic foam are, however, desirable because they give the sensation of a soft touch, a touch which is even more pleasant if a velour type cloth is, for example, used as the surfacing sheet. The inventors have therefore suggested making composite panels which comprise a surfacing layer of synthetic foam 3 to 10 mm in thickness and having thinned rigid portions which are of a clearly defined thickness and which are essential to satisfactory positioning of the headlining and accessories, for example a roof light or a grab handle.
It is also known to produce composite panels by two-pass molding processes. In these processes the first pass is a molding to shape the layer of mineral fibers. The second is a surfacing of this molded layer of mineral fibers, that is to say the gluing-on of the surfacing layer and possibly of the finishing appearance sheet. Molding can then be carried out at a very high temperature, for example approximately 250.degree. C.
This permits of complete and very rapid polymerization of the resin binding the mineral fibers while the surfacing is carried out at 120.degree. C., for example, avoiding any damage to the surfacing layer which is, for instance, of synthetic foam or of the finishing appearance layer which is, for example, a woven fabric.
The insert, that is to say the layer of mineral fibers of the form obtained after the first phase of the process, preferably has thin edges. This thinning of the edges is achieved very easily by appropriate shaping of the mold and of the counter-mold, corresponding to localized increases in the pressure forces exerted and the creation of areas of greater density. In practice, the thinned edges have a thickness of around 1.5 mm. Lesser thicknesses require a considerable increase in the closure pressure applied to the mold. This increase, on the one hand, entails technological difficulties, and, on the other, may lead to the onset of crumbling of the edges due to deterioration of the mineral fibers.
This two-pass process makes it possible to obtain an insert having thin edges which are slightly wider than those obtained by a single-pass process but it does not resolve the problem posed by thick surfacing layers which resume their volume once the panel has emerged from the press. Furthermore, the surfacing layer and the covering sheet are cut larger than the insert to avoid excessively meticulous centering and therefore they overlap the insert.
The simplest answer to this situation is to make a clean cut around the surfaced insert, leaving the edge to reveal both the very dense layer of mineral fibers and the layer of foam and additionally the surfacing sheet. This elementary embodiment results in a product which is fairly unattractive, which has insufficiently thin edges, and which may deteriorate rapidly if the layer of foam remains exposed.
On the other hand, more aesthetic edges are obtained after an edging process. In this case, the composite panel is partially cut out in order to achieve a clean edge over the insert part of the panel and then the intermediate layer and the surfacing sheet are folded back over to the back of the composite panel. Therefore, the edge too is covered with a surfacing fabric and can therefore be left visible. Unfortunately, once the intermediate layer assumes a thickness of 3 to 4 mm, panels are obtained which have on the edges beads of around 1 cm thickness, which are more or less compressible and which are not suitable for a headlining installation.
There is therefore a need for a solution to the above discussed problems in the art of making composite panels which can be used as interior headlinings for motor vehicles.